Shape memory alloys (SMAs) are a class of metal alloys which may undergo near reversible shape changes so that they may repeatedly be alternated between two shapes for, typically, hundreds of thousands of cycles, without failure. The basis for this behavior is the ability of these alloys to adopt two crystal forms, a higher temperature, relatively strong and stiff form called austenite and a lower temperature, somewhat weaker and more compliant form known as martensite. Transformation from one phase to another induces a strain, typically of less than about 8% or so, which is manifested as a change in length of a shape memory alloy sample as it transitions form one phase to the other.
Shape memory has been observed in a number of alloys including Cu—Zn—Al, Cu—Al—Ni, Ti—Nb, Au—Cu—Zn, Cu—Zn—Sn, Cu—Zn—Si, Ag—Cd Cu—Sn, Cu—Zn—Ga, Ni—Al, Fe—Pt, Ti—Pd—Ni, Fe—Mn—Si, Au—Zd, and Cu—Zn but only a few of these alloys are commercially available. Nitinol, an alloy of nickel and titanium in substantially equiatomic proportion, enjoys the widest use. In most commercial SMA alloys the transition from one crystal structure to the other occurs at temperatures which may range from about −100° C. to +100° C.
In SMA devices, shape changes are effected by a change in temperature sufficient to induce transformation. Often the temperature change is promoted by joule heating induced by passing a controlled electric current though the SMA. During this temperature-driven shape change, the SMA may generate appreciable force and SMAs, commonly in the form of linear elements such as wires and analogous geometries like strips, cables, braids and chains, may be employed as active elements in actuators of various kinds. As an example of the forces which may be developed, a single Nitinol wire, 0.008 inches in diameter may develop a force in excess of one pound. Larger diameter wires or wire bundles may generate greater forces in proportion to the cross-sectional area of the SMA.
Transformation from one crystal structure to another occurs over a temperature range usually of less than 20° C. bounded by a transformation ‘Start’ temperature and a transformation ‘Finish’ temperature. The nature of the transformation, martensite to austenite or austenite to martensite is indicated by the resultant phase. Thus the term ‘austenite start temperature’, generally abbreviated as As, would indicate that martensite was transforming to austenite. While an SMA is in the transformation range, for example in the temperature range between As and the austenite finish temperature, Af, its properties may be approximated by a volume-weighted average of the properties of the individual phases. Hence that transformation promotes a generally continuous and smoothly-varying transition from the properties of the transforming phase to the properties of the transformed phase.
SMA-powered actuators are preponderantly employed in an on-off mode. Typically an SMA wire, or analogous geometry tensile force-generating device, biased by, for example, a spring or similar structure in series with the wire, is maintained in its martensitic crystal structure and deformed by the spring to some suitable predetermined length. This configuration may be maintained indefinitely provided the wire temperature is maintained below the temperature at which austenite is the stable phase. If the wire temperature is increased above the temperature where austenite is the stable phase, Af, for example by passing an electrical current through the wire, the wire will contract. By appropriate choice of biasing spring the force applied by the wire will overcome the force exerted by the biasing spring and contract. By suitably securing one end of the wire and attaching the other end to, for example, a plunger, the plunger may be retracted. When passage of electric current is terminated, the wire will cool to a temperature where only martensite is stable and the wire, on transforming to martensite, will be deformed by the biasing spring and extended to its original length. This process cycle may be repeated for many hundreds of thousands of cycles and is typically performed open loop with no feedback to confirm operation of the actuator. This open loop on-off mode of operation limits the number of applications for such SMA-powered actuators over that of more common mechanical, electrical or electro-mechanical actuators which enable progressive actuation and thus allow for operation under proportional control.